Galatians 5:17 English Standard Version (ESV) For the desires of the flesh are against the Spirit, and the desires of the Spirit are against the flesh, for these are opposed to each other, to keep you from doing the things you want to do.
Willful sin is the equivalent of not taking your medicine prescribed by Christ for our sin sickness.
The issue here is what we want to do. We did what we wanted in our past lives which placed us in a position to need Christ. Now that we have Christ we must not act as if we do not need Him.
Psalm 147:13 English Standard Version (ESV) For he strengthens the bars of your gates; he blesses your children within you.
Philippians 4:13 English Standard Version (ESV) I can do all things through him who strengthens me.
We will find His help in both the Old Testament and the New Testament. We must not leave one kind of medicine in the cabinet just because we do not like the taste of it.
Romans 8:28 English Standard Version (ESV) And we know that for those who love God all things work together for good,[a] for those who are called according to his purpose.
Footnotes [a] Some manuscripts God works all things together for good, or God works in all things for the good
Whichever way you prefer to take it, our medicine is meant for good. What we should focus on is the love we have for God. When we were young our mothers gave us medicine and we took it because we trusted her. It is not that much different with God. If we love Him we must trust Him.
Isaiah 46:10 English Standard Version (ESV) declaring the end from the beginning and from ancient times things not yet done, saying, ‘My counsel shall stand, and I will accomplish all my purpose,’
Do we have to understand how this works to believe it does work? Does our lack of understanding hinder our belief? Does unbelief hinder our trust? Does lack of trust hinder our ability to do what is asked of us?
Faith answers all those questions for us.
Walking by faith does not need to know purpose. It only requires us to trust what God has built up in us. Did the medicine work? Did it accomplish His will by taking it with faith?